1. Field of the Invention
The present invention relates to an image pickup apparatus, such as a video camera, and more particularly to an image pickup apparatus equipped with an ND filter for adjusting the amount of light incident on an image pickup element from the aperture of a diaphragm mechanism.
2. Description of the Related Art
Recently, the cell size of a CCD sensor has been reduced according to reduction of the size of a video camera, and this makes image quality liable to be adversely affected by light diffraction. This results in a decrease in the upper limit of the F number which is indicative of a degree of stopping down of the aperture of a diaphragm mechanism that controls exposure. To reduce adverse influence of the light diffraction on image quality even by a slight degree, differently from the conventional technique in which the diaphragm mechanism and an ND (Neutral Density) filter attached to the diaphragm mechanism are simultaneously controlled, there has been proposed a technique which controls the diaphragm mechanism and the ND filter independently of each other, thereby making it possible to reduce the amount of light with no need to stop down the aperture of the diaphragm mechanism.
FIG. 7 shows an example of an image pickup apparatus that controls the ND filter independently of the diaphragm mechanism.
The image pickup apparatus is comprised of a lens 110, the diaphragm mechanism 120 for controlling the amount of light incident thereon from the lens 110, a CCD sensor 200 for photoelectrically converting light incident thereon from the diaphragm mechanism 120, and the ND filter 160 for limiting the amount of the light incident on the CCD sensor 200 from the diaphragm mechanism 120.
The diaphragm mechanism 120 is driven by a diaphragm-driving motor 130. The diaphragm-driving motor 130 is driven by a diaphragm mechanism-driving device 140. Further, a degree of stopping down of the diaphragm mechanism 120 is detected by a diaphragm mechanism sensor 150.
The ND filter 160 is driven by an ND filter-driving motor 170. The ND filter-driving motor 170 is driven by an ND filter-driving device 180. Further, a state of covering of the diaphragm mechanism 120 with the ND filter 160 is detected by an ND filter sensor 190.
The CCD sensor (image pickup element) 200 is controlled by an image pickup element-driving device 210. The image pickup element-driving device 210 reads out a photoelectrically converted signal from the CCD sensor 200, gives to the CCD sensor 200 a so-called electronic shutter function for controlling a time period over which the signal is stored, and controls the electronic shutter function. The signal photoelectrically converted by the CCD sensor 200 is sampled and electrically amplified by a CODS (Correction Double Sampling/AGC (Automatic Gain Control) 220. An analog signal output from the CDS/AGC 220 is converted to a digital signal by an A/D converter 230, and the digital signal is delivered to a DSP 240.
The DSP 240 is a signal processing device equipped with control functions for performing gamma correction on the digital signal received from the A/D converter 230, carrying out processing concerning color separation, color difference matrix, and the like, on the gamma-corrected signal, then adding a synchronization signal to the signal, to thereby generate a standard television signal. The DSP 240 receives a processing command from a microcomputer 270 that controls the overall operation of the image pickup apparatus. Image data processed by the DSP 240 is stored in a memory 250, while image information e.g. on picked-up still images and moving images is recorded in a recording medium 260, such as a memory card. It should be noted that in FIG. 7, reference numeral 280 designates a liquid crystal panel, and reference numeral 290 designates a moving-image/still-image changeover switch.
Next, a description will be given of a method of controlling the diaphragm mechanism 120 and the ND filter 160.
First, light incident from the lens 110 passes through the aperture of the diaphragm mechanism 120 and the ND filter 160, and light limited by the ND filter 160 enters the CCD sensor 200. A signal photoelectrically converted by the CCD sensor 200 is converted to a digital signal by the CDS/AGC 220 and the A/D converter 230, and is subjected to camera signal processing by the DSP 240. The DSP 240 outputs luminance data to the microcomputer 270 according to a corresponding range for exposure control, and the microcomputer 270 performs computation for the exposure control based on the luminance data. If it is determined based on the result of the computation that proper exposure is not obtained, the diaphragm mechanism 120, the ND filter 160, the electronic shutter function of the CCD sensor 200, and the AGC of the CDS/AGC 220 are controlled such that proper exposure can be obtained.
Out of control devices related to the above four control parameters for exposure control, the diaphragm mechanism 120 and the ND filter 160 will be described with reference to FIG. 8.
First, at (A) in FIG. 8, the aperture of the diaphragm mechanism 120 is opened, and the ND filter 160 is fully retracted from the aperture of the diaphragm mechanism 120. From this state, exposure is controlled in a direction reducing the amount of light incident on the CCD sensor 200. More specifically, as shown at (B) in FIG. 8, when the diaphragm mechanism 120 is stopped down to a certain aperture diameter (F4.0), the aperture diameter of the diaphragm mechanism 120 is fixed, whereafter the ND filter 160 is continuously and progressively inserted into the aperture for control of the exposure. It should be noted that “the ND filter is inserted into the aperture” is intended to mean “the ND filter is inserted into an optical path immediately after or immediately before the aperture (in the present example, “immediately after the aperture”) which has the same diameter as that of the aperture, to be exact, and “the ND filter is retracted from the aperture” is intended to mean “the ND filter is retracted from the optical path immediately after or immediately before the aperture (in the present example, “immediately after the aperture”), to be exact. This applies to the description and also to the claims.
At this time, as shown at (C) in FIG. 8, the ND filter 160 is sometimes inserted halfway into the aperture of the diaphragm mechanism 120, and as shown at (D) and (E) in FIG. 8, there sometimes occurs a case where different density areas of the ND filter 160 cover the aperture of the diaphragm mechanism 120 at the same time.
In such cases, as shown in FIG. 10, diffraction of light is caused by a thickness step or a density step existing in the ND filter 160, which results in the degraded resolution of images.
In general, compared with moving image shooting which gives priority to the smoothness of an image, still image shooting gives a top priority to the resolution of an image, and therefore it is preferable to avoid insertion of the ND filter 160 into the aperture of the diaphragm mechanism 120 in such an incomplete fashion as causes a thickness step or a density step to exist in the ND filter 160. To this end, if the use of the ND filter 160 for still image shooting is inhibited, compared with the moving image shooting, the dynamic range of exposure control is reduced by a degree corresponding to the density of the ND filter 160.
To overcome this inconvenience, the following technique has been proposed (see Japanese Laid-Open Patent Publication (Kokai) No. 2004-72580).
According to the proposed technique, in the still image shooting mode, as shown in FIG. 9, the aperture of the diaphragm mechanism 120 is more closed as an object becomes brighter, and the control of the diaphragm mechanism 120 is stopped at an aperture diameter corresponding to an F number of F11. Subsequently, the closing operation of the ND filter 160 is controlled such that the whole aperture diameter of the diaphragm mechanism 120 is covered with the ND filter 160. At this time, the aperture diameter of the diaphragm mechanism 120 is increased to a value corresponding to an F number of F4 to correct light shielded by the ND filter 160 according to the density of the ND filter 160.
Further, a degree of exposure or an amount of light which cannot be corrected by the increase in the aperture diameter is corrected by decreasing the speed of the electronic shutter, in other words, by increasing a time period over which light is stored in the CCD sensor 200. By the correction control, it is possible to make inconspicuous a sudden change in the amount of light, caused by the insertion of the ND filter 160 into the aperture of the diaphragm mechanism 120.
After that, if the brightness of the object further increases, the exposure is controlled by increasing the electronic shutter speed from 1/60 sec. to 1/250 sec., and decreasing the aperture diameter of the diaphragm mechanism 120 from a value corresponding to a F number of F4 to a value corresponding to a F number of F11.
Inversely, exposure control from a state in which the ND filter 160 is fully inserted into the aperture of the diaphragm mechanism 120 to a state in which the ND filter 160 is fully retracted from the opening is performed in a direction in which the object becomes darker, opposite to the direction in which the object becomes brighter, as described above.
More specifically, in the state in which the ND filter 160 is fully inserted into the aperture of the diaphragm mechanism 120, the electronic shutter speed is set to 1/60 sec. and the aperture diameter of the diaphragm mechanism 120 is set to a value corresponding to an F number of F4. Further, if the microcomputer 270 determines that the object has become darker, the electronic shutter speed is set to 1/250 sec. with the aperture diameter of the diaphragm mechanism 120 being set to a value corresponding to an F number of F11, and the ND filter 160 is fully retracted from the aperture of the diaphragm mechanism 120. After that, if the object becomes still darker, the exposure is controlled by the electronic shutter speed and the diaphragm mechanism 120.
As described hereinabove, the ND filter 160 is not continuously inserted into the aperture of the diaphragm mechanism 120, but exposure control is carried out such that the ND filter 160 is fully inserted into or retracted from the aperture of the diaphragm mechanism 120, whereby it is possible to prevent light diffraction from being caused by the density step or the thickness step in the ND filter 160. This makes it possible to prevent degradation of the resolution of images during still image shooting, to secure the dynamic range of exposure control using the ND filter 160 both for still image shooting and for moving image shooting.
However, in the aforementioned Japanese Laid-Open Patent Publication (Kokai) No. 2004-72580, the ND filter 160 is caused to perform an opening or closing operation suddenly on the aperture of the diaphragm mechanism 120, and therefore there exists a luminance shock (sudden change in luminance) on the screen, though for a moment. The luminance shock not only degrades image quality but also becomes a cause of missing a perfect moment to pick up an image. More specifically, when a user is about to pick up a still image, if the ND filter 160 is suddenly and automatically inserted into or retracted from the aperture of the diaphragm mechanism 120, there is a large change in luminance, which can cause the user to hesitate to depress the shutter button to pick up the image.
Although the luminance shock does not actually damage the result of still image shooting insofar as it occurs on a monitor screen used during still image shooting, if the luminance shock occurs during moving image shooting, continuity of luminance is lost.